These two vowel sounds are similar in some ways, but in English they are definitely different. The difference between them may seem small to the ear of a student of English, but the difference in the meaning is big. There are many words that depend on that small difference in the sound.

First, the similarities. They might seem the same to your ear because:
— they are both made with a relaxed tongue,
— they are both in the central part of the mouth (not in the front or the back),
— and they are both made without rounding the lips.

The difference is:
— how high or low the tongue is.

Short-u is in the middle center of the mouth — this is the same as Schwa (see “The Sound of “Schwa”) — the tongue is neither up high nor down low. But for Short-o, the tongue needs to be lower, which means that the mouth needs to be more open.

Try it!
Let’s use the words “fun” and “fawn”. Start with the word “fun”. This word needs Short-u, so the tongue should be relaxed in the middle of the mouth (not high, not low, not front, not back), and do not round your lips: fun.

Now, the next word is almost the same, but the mouth needs to be more open so that the tongue can go down lower: fawn.

That is the difference between “bus” and “boss”.

One other reason that these two sounds might seem confusing is that the Letter “O” sometimes borrows the Short-u sound. For example, the word “love” uses Short-u rather than Long-O or Short-o. There are several frequently used words that do this, such as: “nothing”, “some” and “of”. (See more examples in “The Sounds of O” and in “What is Schwa?”) In addition, there are a few words that look like they should sound the same, but use different vowel sounds: the word “gone” uses Short-o, but “done” and “none” use Short-u.

(Try the tongue twister “Fuzzy Wuzzy” for a fun way to practice the Short-u sound)